Which one should I get?

[[SOE]No.Mam]

all parts from Men of War are stand alone.

Soldiers - Heroes of World War 2: old - great mods (BTRH & SoE...) - easy too learn/play - multiplayer is dead
Faces of War: nice singleplayer - multiplayer cheatable - hitpointsystem
Men of War: good single- and multiplayer - more than 500 players online - cheatfree
Men of War: Red Tide: only singleplayer
Men of War: Assault Squad: last part from series - great multiplayer - skirmish ...

i think you should start with Men of War. this game is good for single- and multiplayer.